1. Field of the Invention
The invention relates to analog-to-digital conversion technologies.
2. Description of the Related Art
Information communication terminals such as cellular phones are equipped with analog-to-digital converters for converting analog signals received from wired or wireless communication lines into digital signals. Information communication terminals are powered by batteries, and technologies for lower power consumption have been intensively developed in recent years.
In some radio communication techniques, modulation schemes are switched depending on changes in the environment. In such radio communications, resolutions required of analog-to-digital converters vary with the modulation schemes. Even with a fixed modulation scheme, the resolutions required of analog-to-digital converters also vary depending on signal, noise, and other conditions. Consequently, when analog-to-digital converters are given a fixed resolution, they can sometimes operate with performance higher than required, wasting power uselessly. For example, Japanese Patent Laid-Open Publication No. 2003-198368 has proposed an analog-to-digital converter which controls its power consumption to change performance.
Nevertheless, when a variable current source is used to adjust the amount of the current supplied to a preamplifier of the analog-to-digital converter, the adjustable range is narrow since the MOS transistors in the current mirror circuit must stay in their saturation regions. Considering the tendency toward lower power consumption and the future trend toward lower device voltages, the adjustable range will become even narrower. With increasing speedup, the power consumption of digital blocks within an analog-to-digital conversion apparatus, including a clock tree, becomes higher in proportion than that of the constant current passed through the preamplifier.